如图，质量为5kg的物体，在一个F=25N的水平外力作用下，沿墙壁匀速下滑.如图,试分析：
（1）物体受到几对平衡力的作用？请在图中画出物体所受到的摩擦力、重力的示意图．
（2）物体所受的摩擦力有多大？（g取10N/kg）
（1）两对（2分），如图
（2分） （2）50N
题目分析：（1）物体在竖直方向上受到的平衡力：竖直向下的重力和竖直向上的摩擦力；在水平方向上受到的平衡力：水平外力作用和墙对物体的支持力；因此物体受到二对平衡力；
过物体的重心画出竖直向下和竖直向上大小为50N的重力和摩擦力，如图所示：
（2）
，
因为摩擦力和重力是一对平衡力，因此摩擦力的大小等于50N．
点评：关键是根据运动状态得出受平衡力，以及受到两对平衡力的作用，水平方向与竖直方向的力没有直接关系，一定要搞清楚。
Korean culture is really popular right now. The Korean Wave(韩流) is sweeping Asian countries including(包括) 1 . Young people are 2 in Korean TV plays, Korean pop songs and the Korean language. The Korean Wave started 3 years ago with the TV series(电视系列剧) Winter Sonata(冬季恋歌). This love story is still popular now. People, especially girls, like the beautiful stories and handsome actors 4 Pei Yongjun ( 裴勇俊). In the music world, Korean stars are making 5 heard in China. You can often find big Korean names such as Baby Vox, SES and Finkle at the top of the Chinese 6 charts(排行榜). The Korean Wave 7 makes young people try the clothes and hairstyles of the Korean pop stars. Besides those, now some girls in China are having plastic surgery(整形手术) 8 their looks. It is said that many Korean stars look beautiful 9 plastic surgery. As you see, Korean culture has changed us a lot. Is it 10 for us to follow in every way? We each should think about it.
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